Abstract: A method of manufacturing an optic transmission module assures a high reliability for the module by inspecting the character of a low bit error rate (BER) of the module quickly and accurately. The module is employed in an optical communication system (light transmission system) required to cope with information communication systems that are becoming faster and faster in operation and larger and larger in capacity. Also disclosed is a system and a method for inspection of the BER character which uses a rectangular wave pulse as an interference light to calculate a degradation quantity of a signal-to-noise ratio (S/N) on the basis of a simple theory, thereby inspecting the BER character by extrapolating a case in which there is no interference light (S/X=∞ or X=0) with use of a sign error rate theoretical value according to a bit error rate of a light signal measured according to the degradation quantity.